1. Once a day at most, but I sometimes go 2-3 days without jerking off. It depends on how busy I am, my general stress levels, etc. Overall, probably 4-5 times a week, which is definitely more than pre-T.
2. It's happened to me in public but is honestly pretty uncommon, and seems driven more by boredom than anything else. I've gotten spontaneous boners while sitting in traffic, for example, but never (or least very rarely) at work.
3. It just feels like arousal, as others have said. I'd say it's a more obvious (and hardening) sensation than what I used to experience pre-T, but yeah, it's difficult to get more specific than that.